It is a general practice that products are assembled while they are being moved on a conveying device along a production line. The instructions of the procedure to be followed downstream along the line are conveyed by writing them on a sheet, which is attached on each product (work). Workers confirm their actions by reading the instructions, which list required operations (refer to, for example, Japanese Laid-Open Patent Publication No. 2003-6287 (pages 2-4, FIG. 1)).
However, if such an instruction sheet has dropped off from a work while the work is being conveyed along the production line, then the workers positioned downstream cannot determine what operations are required on the work. As a result, if workers miss some of the required operations, then missed operations are detected, for example, by an inspection on the work executed after the supposed completion. In this case, because of the missed operations, the work is returned for a reprocessing, which causes a problem of delay in the manufacturing process, resulting in a decrease in operational efficiency.